Typically, mineral values have been produced by conventional mining and milling methods. In situ leaching is an attractive alternative for mineral values recovery from ore deposits beneath the water table or too deep for strip mining. The in situ leaching of mineral values from subterranean deposits is well known in the art as a practical and economical means for recovering certain values such as uranium, copper, nickel, molybdenum, rhenium, vanadium and the like. Basically, in situ leaching is carried out by injecting into the subterranean deposit a leaching solution which will solubilize the mineral value desired to be recovered. Conventionally, the leaching solution is brought into contact with the subterranean deposit by injection into one or more injection wells which penetrate the deposit. The leaching solution is introduced into the injection well under sufficient pressure to force it out of the well bore into the adjacent deposit. Continued injection of leaching solution drives the solution through the deposit to one or more spaced-apart production wells where the solution is recovered for subsequent extraction of the mineral values. The number of injection and production wells and the spacing therebetween can vary depending upon the nature of the formation. Additionally, the pattern of injection and production wells can also vary although a typical pattern is the five-spot pattern consisting of a centrally disposed recovery well and four injection wells spaced around the recovery well. Alternatively, a given volume of leaching solution can be injected into a well to percolate into the surrounding formation. Following the injection phase, the well is pumped out and the injected leaching solution is recovered from the same well into which it had been injected.
The first essential step in most leaching operations is to oxidize the mineral values to a form which is soluble in the leaching solution. For example, an essential step in uranium leaching is the utilization of an oxidizing agent to contact the mineral deposits to oxidize the uranium from its insoluble quadrivalent form to its soluble sexivalent form. The deposit is then contacted with the leaching solution to solubilize the sexivalent uranium which is extracted with the solution. The oxidation can be carried out as a separate step or simultaneously with the leaching step by dissolving the oxidizing agent in the leaching solution.
In addition to the mineral values, other oxidant-consuming species such as sulfur compounds, mainly present in the form of iron sulfides, and carbonaceous material are present in the subterranean deposits. Sulfides are usually present in concentrations far exceeding those of the other two materials and, therefore, represent the major oxidant-consuming species. For example, in an in situ uranium leaching process, the sulfides preferentially consume the oxidant available in the injected lixiviant, thus effectively inhibiting the solubilization of uranium until most or all of the scavengers are oxidized. In many formations this scavenging or reducing capacity is so high that the leaching rate is limited by the supply of oxidant. This is particularly true where oxygen is used as the oxidizing agent because of the limited solubility of oxygen in a leaching solution; the scavenging of the oxygen supply is most marked at early stages of the leaching operation. In the typical in situ leaching operation, where the lixiviant, or leaching solution, is injected into one well and the pregnant lixiviant, or leachate, is produced from other wells spaced at a distance, no uranium will be produced in the pregnant lixiviant until the entire formation is essentially oxidized.
It is also well known to those skilled in the art that a complication in the in situ extraction of mineral values by oxidative treatment of a carbonaceous matrix is the deposition of sulfur. The formation of elemental sulfur can block the pore channels or lodge in pore throat openings thereby inhibiting the production of the enriched leach solution.
The present invention overcomes the foregoing deficiencies by providing a process for the recovery of mineral values from subterranean formations which contain sulfur where the loss of permeability and oxidant is substantially reduced.